Google's self-driving cars have clocked up more than 1.3 million miles in California and Texas since testing began in 2012, and Google says they have never been responsible for an accident.

However, the report reveals that humans have had to intervene 13 times to avoid an collision between September 2014 and November 2015, and in another 69 cases, the driver had to take control to prevent dangerous driving.

In 272 cases, meanwhile, drivers had to take the wheel to deal with "software failures" such as failing to perceive obstacles or not anticipating pedestrians crossing the road.

How Google's cars see the road Photo: Google

The figures show that while billions are being invested in driverless cars, the technology is still far from complete. However, Google says that its drivers "err on the side of caution and take manual control if they have any doubt about the safety of continuing in autonomous mode."

The company also said it has made great strides in just 15 months in reducing how frequently humans are required to intervene. For example, before November the cars had not needed human control to prevent an accident for six months.

"As we continue to develop and refine the self-driving software, we are seeing fewer disengagements of this type despite a growing number of miles driven each month. The number of autonomous miles we are driving between immediate manual control disengagements is increasing steadily over time," the report said.

Chris Urmson, the head of Google's driverless car project, wrote: "This trend looks good, and we expect the rate of these incidents to keep declining.

"Although we’re not quite ready to declare that we’re safer than average human drivers on public roads, we’re happy to be making steady progress toward the day we can start inviting members of the public to use our cars."

Humans actually take the wheel of Google's cars thousands of times, although most of these are routine and were not included in the report.

Despite the incidents, Google and many proponents of driverless cars believe that the technology is inevitable and will be many times safer than human vehicles.

A recent academic study of self-driving car accidents has found that driverless cars are objectively safer than regular ones.